1. Field of the Invention
The present invention relates to a tone generating apparatus for use in electronic musical instruments, such as a synthesizer, an electronic piano, an electronic organ and a single keyboard. More particularly, this invention pertains to a tone generating apparatus which is designed to need fewer oscillators without reducing the number of polyphonic sounds (simultaneously generating sounds).
2. Description of the Related Art
Conventional tone generating apparatuses for use in electronic musical instruments each have multiple digital control oscillators (DCOs) as tone generating sources. These DCOs are properly and selectively combined and driven in accordance with, for example, the timbre designated through an operation panel, or the tone range specified through a keyboard.
Such a tone generating apparatus is designed to simultaneously drive three oscillators when one key of a keyboard instrument is depressed. The individual oscillators produce the respective three tone components of a soft-hit component, a hard-hit component and a striking component. These tone components are combined to provide a single tone signal. This design is employed to ensure generation of musical tones close to sounds of a natural musical instrument.
The "soft-hit component" is a tone component whose frequency varies in proportion to the pitch and which has a relatively long tone-generating time or tone-ON time. The "hard-hit component" is a tone component whose frequency varies in proportion to the pitch and which has a relative short tone-ON time. The "striking component" is a tone component whose frequency should not necessarily be proportional to the pitch while having a shorter tone-ON time.
The ordinary tone generating apparatus is equipped with the same number of tone generating circuits each having the mentioned three oscillators as the number of polyphonic sounds.
As described above, in order to generate musical tones of hard-hit and striking components having a short tone-ON time, the conventional tone generating apparatus is provided with the same number of oscillators for each of the hard-hit and striking components as the number of oscillators for the soft-hit component having a long tone-ON time (the number of polyphonic sounds). At the time a musical tone is generated, channels are assigned to the oscillators for the individual tone components to generate the individual tone components simultaneously.
However, the hard-hit component is generated for a shorter period than the soft-hit component and has a shorter attenuation time, while the striking component does not vary in accordance with the pitch, i.e., the position of a depressed key and will attenuate immediately after it is generated for a short period of time.
To provide the same number of oscillators for generation of the hard-hit component or striking component that is generated only for a short period of time as the polyphonic number and assign a channel to the tone generation requires a great amount of hardware and results in a complicated structure. This design also impedes to the efficient use of the oscillators. These shortcomings inevitably make expensive electronic musical instruments to which the tone generating apparatus is applied.